var phoneWidth = parseInt(window.screen.width);
var phoneScale = phoneWidth/640;
var ua = navigator.userAgent;
if (/Android (\d+\.\d+)/.test(ua)){
    var version = parseFloat(RegExp.$1);
    // andriod 2.3
    if(version>2.3){
        document.write('<meta name="viewport" content="width=640, minimum-scale = '+phoneScale+', maximum-scale = '+phoneScale+', target-densitydpi=device-dpi">');
        // andriod 2.3以上
    }else{
        document.write('<meta name="viewport" content="width=640, target-densitydpi=device-dpi">');
    }
    // 其他系统
} else {
    document.write('<meta name="viewport" content="width=640, user-scalable=no, target-densitydpi=device-dpi">');
}